There is a large amount of albumin in blood. It is known that since albumin in blood binds to a peptide in blood to form a complex, albumin interferes with the detection of peptide in blood. Therefore, there is a need to separate albumin from a peptide, namely to liberate a peptide from a complex of albumin and peptide. For example, US Patent Application Publication No. 2012/277407 describes that a liquid sample containing a complex of peptide and albumin undergoes heat treatment, and a self-aggregate of albumin is formed in the liquid sample, so that the peptide is liberated from the complex.
As described in US Patent Application Publication No. 2012/277407, the present inventors have found that a liquid sample containing a complex of peptide and albumin undergoes heat treatment, and a self-aggregate of albumin is formed in the liquid sample, so that the peptide is liberated from the complex (US Patent Application Publication No. 2012/277407 is herein incorporated by reference). It has been considered that the self-aggregate of albumin almost loses the ability to bind to peptides due to denaturation of higher order structure of albumin upon heat treatment. However, the present inventors have found that some of the peptides are not liberated by the heat treatment and are incorporated into the self-aggregate of albumin. Accordingly, there is a demand for the development of a means for liberating a peptide incorporated into a self-aggregate of albumin.